The invention relates to the regulation of the cleaning temperature of pyrolytic ovens whose base plate heating element is outside the cooking chamber.
The cleaning of the cooking oven by pyrolysis is a known operation which consists in destroying the dirt due to splashes on the walls which occur during cooking, by bringing the oven and the dirt to a high temperature.
Pyrolytic cleaning requires temperatures of the order of 400.degree. to 500.degree. C., but it is necessary to reach a temperature of the order of 500.degree. C. for the operation to be effected completely in an acceptable time justifying the adoption of this method. Further, the present state of the art limits the maximum temperature which can be withstood by enamel-lined walls having all the necessary characteristics of a cooking oven to about 520.degree. to 550.degree. C., that is unless the quantities of parts concerned justify the provision of a special high temperature vitrifying oven for the sole manufacturing operation of applying a refractory enamel having a higher vitrification temperature.
The result of this is that to clean effectively in a shorter time, e.g. one and a half hours as a function of the degree of dirtying, the temperature of the walls of the furnace must be as homogenous as possible as soon as 500.degree. C. is approached. This is obtained relatively easily when the heating elements of the oven are situated inside the heat-proof chamber.
However, this no longer applies when the base plate heating element of the oven is situated outside the oven chamber, for the following reasons: for cost price reasons the base plate elements used for heating during cooking have a heat emission rate (power per unit area) which is fairly high, so that local overheating of the wall occurs in the immediate vicinity of the heating element, this not being a disadvantage as long as the temperature is sufficiently far from pyrolysis temperatures. A known solution, in the U.S. Pat. No. 3,121,158 consists in reducing, for the pyrolysis operation, the emission rate of the hearth heating element by connecting it in series with another heating element, but, to obtain the power necessary for a sufficiently rapid rise to the required temperatures, one or several extra heating elements whose use is reserved for cleaning operations must then be added at the front edge of the oven liner and around the door.